In a typical cellular radio system, wireless communication devices communicate via a Radio Access Network (RAN) to one or more core networks. The radio access network covers a geographical area that is divided into cell areas, with each cell area being served by a radio access node. A wireless communication device within the cell may communicate with the radio access node over the air interface.
Under certain circumstances, it may be necessary to use access control to control the load of the network. For example, access control may prevent at least some of the wireless communication devices from accessing the network for the purposes of (a) congestion control, (b) emergency situations, and (c) other special situations, such as Public Land Mobile Network (PLMN) failure. 3GPP TS 22.011, “Service Accessibility,” describes a method of access control that varies the probability of wireless communication devices accessing the network.
As an example, an access control technique may assign each wireless communication device to one out of ten randomly allocated mobile populations, defined as Access Classes 0 to 9. The population number is stored in the Subscriber Identity Module (SIM)/Universal Subscriber Identity Module (USIM). In addition, wireless communication devices may be members of one or more of 5 special categories (Access Classes 11 to 15), also held in the SIM/USIM. These classes are allocated to specific high priority users.
Access Class Barring (ACB) is used to control the access attempts of wireless communication devices belonging to any of the above access classes. Especially, access of wireless communication devices in classes 0-9 can be limited, whereas wireless communication devices belonging to the special classes can be granted direct access overriding the ACB applied to the normal classes 0-9.
In UTRAN, for example, the existing ACB mechanism is based on a bitmap broadcasted by the network. The bitmap indicates which of classes from 0 to 9 are barred and which are not. For example, there is an Access Class Barred information element in specification 3GPP TS 25.331, “Radio Resource Control (RRC); Protocol Specification.”
In E-UTRAN, the ACB mechanism is based on an access barring factor and an access barring time, both of which are broadcast in the system information when ACB is applicable. When ACB is used, wireless communication devices attempting access will make the barring check. First, the wireless communication device draws a uniform random number and compares it against the access barring factor to determine whether access is barred or not. A value lower than the access barring factor denotes that the wireless communication device is barred, otherwise access is allowed. If barred, the wireless communication device proceeds to determine the time the barring is applicable.
3GPP TS 22.011, “Service Accessibility,” also describes Service Specific Access Control (SSAC) for MultiMedia TELephony service (MMTEL) functions. The purpose is to apply independent access control for telephony services (MMTEL-voice, MMTEL-video) for mobile originating session requests from idle-mode. For example, a service probability factor and mean duration of access control may be assigned for each of MMTEL voice and MMTEL video. Finally, Access Control for Circuit Switched Fall Back (CSFB) is defined.
Examples of procedures for establishing a voice call during ACB and SSAC are set forth in TS 36.331, “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol Specification” and TS 24.173, “IMS Multimedia telephony communication service and supplementary services; Stage 3.” In general, the RRC layer executes the ACB barring check whereas the Internet protocol Multimedia Subsystem (IMS) layer executes the SSAC barring. The procedure may include the following steps:
1. The wireless communication device reads ACB and SSAC parameters broadcast by the radio access node based on the normal System Information Acquisition procedure.
2. The IMS layer in the wireless communication device triggers a voice call. It requests the RRC layer to forward SSAC parameters to the IMS layer in the wireless communication device.
3. Upon request from the higher layer (i.e., the IMS layer), the RRC layer forwards SSAC parameters to the higher layer.
4. The IMS layer in the wireless communication device determines if the call is barred or not.
a. If the call is barred, the call establishment procedure stops.
b. If the call is not barred, a new call is generated. This can be seen as a new RRC connection establishment in the RRC layer. During RRC connection establishment, the wireless communication device checks ACB parameters. If the wireless communication device is barred according to ACB parameters, then it is not allowed to access the network.
As can be seen from the procedure above, the wireless communication device might be barred with ACB even though the access is allowed with the SSAC.